Isomerization of xylene



ISOMERIZATION F XYLENE States Patent Roger W. Hill, El Cerrito, and Raymond V. Luthy, Berkeley, Calif., assignors to California Research Corporation, San Francisco, Calif., a corporation of Delaware This invention relates to the isomerization of aromatic hydrocarbons and pertains more particularly to the catalytic isomerization of xylene mixtures containing ortho meta-, and para-xylene.

Para-Xylene is currently being separated from mixed xylene isomers in commercial quantities by fractional crystallization. The feed to a crystallization unit may be a xylene fraction separated from catalytically reformed naphtha by careful fractional distillation. Such a fraction will ordinarily contain from -10% of non-aromatic hydrocarbons and 5-15% lethylbenzene, in addition to ortho, meta, and para-xylene. In some instances, the feed to the crystallization unit is prepared by a combination of extractive distillation and careful fractional distillation andV is substantially free of non-aromatic hydrocarbons. In either type of feed, the C8 aromatic isomers are present in the following proportions:

Percent by volume Ortho-,xylene 20-27 Meta-xylene 50-54 Para-xylene 20-25 Ethylbenzene 5-12 These proportions represent the equilibrium composition of a C8 aromatic fraction produced by dehydrogenation of naphthenic hydrocarbons, and the precise composition of a given equilibrium mixture depends upon the temperature at which the aromatic hydrocarbons are produced. The above ranges of composition hold for temperatures inthe range of 600 F. to l000 F., orthoxylene and ethylbenzene contents increase as temperature is raised, and meta-xylene and para-xylene contents decrease as temperature is raised.

Only about one-half of the para-xylene contained in these fractions can ordinarily be separated by fractional crystallization. In order to obtain incremental production of para-xylene, it is desirable to isomerize the mother liquor produced in the fractional crystallization process.

It has now been found that these mother liquors can be advantageously isomerized to produce para-xylene with a minimum loss of xylene due to disproportionation and cracking by fractionally distilling the mother liquor to separate an overhead fraction comprising metaand para-xylene and having an ortho-xylene content substantially less than that of the mother liquor and Ya bottoms fraction having an ortho-xylene content substantially higher than that of the mother liquor, contacting the overhead fraction with a silica-alumina catalyst at 700 F. to 1000 F. at a liquid'space velocity in the range i from 0.5-5.0, and separately contacting' the ortho-xylene rich bottoms fraction with a silica-alumina catalyst at a temperature in the range from 900-1050 F. at a liquid space velocity in the range from 0.5-5.0 v./v./hr. The

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' 2 ortho-xylene rich fraction is subjected to isomerization conditions substantially more severe than those employed in treating the overhead fraction, higher temperature or lower space velocity, or both being employed to isomerize the ortho-xylene rich fraction.

It is an object of the present invention lto carry outA the isomerization of a mixture of ortho, meta, and paraxylene to produce increased yields of para-xylene.

It is a further object of the present invention to carry out the same isomerization with a minimum xylene loss dueto cracking and disproportionation.

Pursuant to this invention, a.xylene feed is passed into a fractional crystallization zone where portions of the para-xylene isomer are crystallized at low temperatures. The mixture is then introduced into a centrifuge from which the crystallized para-xylene is removed as a cake',- and the mother liquor is removed as the filtrate. Themother liquor, now deficient in para-xylene in respect to the xylene-isomer equilibrium, is passed into a distillation zone to remove a major proportion of the ortho-xylene and hydrocarbons with higher boiling points than orthoxylene from the mother liquor. The separation is preferably efect'ed by super-fractionation in a distillation column. One equivalent to at least about 35 theoretical plates appears necessary for practical operation; desirably about 45 and preferably about 60 theoretical plates are utilized. By etlicient fractionation, an overhead fraction comprising meta-xylene, para-Xylene, ethylbenzene, and a small proportion of ortho-xylene is separated from a bottoms fraction composed predominantly of orthoxylene. v

The overhead fraction is passed into an isomerization zone where it is contacted with a silica-alumina catalyst at a temperaturein the range about 700 F. to l000 F. The overhead fraction is contacted with the catalyst at a space velocity in the range 0.5-5.0 L. H. S. V. (liquid hourly space velocity), or 0.5 to 5.0 volumes of liquid feed per volume of catalyst per hour.

The bottoms fraction, predominantly ortho-xylene, is separately passed into another isomerization zone and contacted with a silica-alumina catalyst at a temperature in the range about l900 F. to 1050 F., and a space velocity in the range from 0.5-5.0.

The efficiency of the isomerization treatment is indicated by the para-xylene` content of the product. The more closely the para-xylene content of the product apv proaches the para-xylene content of an equilibrium mixture of Xylenes at the isomerization temperature, the more efficientA is the process.` A convenient numerical index of eiiiciency is obtained by calculating the percent approach to paraxylene equilibrium content as follows: y

Percent p-xylene equilibrium approach# percent p-xylene percent p-xylene in productl in feed percent p-xylene percent p-xylene at etjluilibriurnV in feed the weight of para-xylene present by the total weight of only-the three isomers, 'ortho-, meta-, and para-xylene present. The theoretical xylene equilibrium values 'used herein are'those reported' by Rossini et al., and are given in Table 1.

TABLE 1 EQUILIBRIUM CONCENTRATIONS OF THE C ALKYLBENZENES l Composition, in mole fraction, oi equilibrium mixture oi isomers Temperature, F. Ethylbenzenc o-Xylcne m-Xylenc p-Xylene C3 Xylene rCr Xylene C A Xyiene C; Xylene Basis Basis Basis Basis Basis Basis Basis Basis l Rossini t al.; `Tour. of Research Natl Bureau of Standards, RP 1732, vol. 37, p. 11.5,

August 194 It has been found that the isomerization of a xylene mixture in which the ortho-xylene concentration is high requires drastic conditions of temperature or. Contact time, or both, to obtain a high percent approach to para xylene equilibrium while xylene mixtures predominating in meta-xylene can be isomerized at relatively mild conditions to produce a product having a para-xylene content closely approaching equilibrium content. The isomerization of predominantly ortho-xylcne feeds results in greater losses due to disproportionation and cracking than the isomerization of a feed predominantly meta-Xylene since stock losses proceed more rapidly at severe conditions. The effects of. xylene isomer concentration in the feed and the temperatures employed in the isomerization step arc shown by the followingy examples:

Example 1 A 99% ortho-xylene feed is isomerized by contacting it with a silica-alumina catalyst at a temperature of 800 F. and a L. H. S. V. of l. The percent approach to para-xylene equilibrium is 3.0% with a xylene loss of 5.0%.

Example 2 v,The Example l is repeated except the isomerization is carried out at a temperature of 1000" F. and an L. H. S. V. of 2. The percent approach to para-xylene equilibrium is 90%,with a xylene loss of 11%.

Example 3 l A A 95% meta-xylene feed is isomerized by contacting it with a silica-alumina catalyst at a temperature of 800 F. and a L. H. S. V. of 2. The percent approach to para-xylcne equilibrium is 95% with a xylene stock loss of less than 1%.

Example 4 `90%, but that xylene losses due to cracking and disproportionation also increase from 5% to 11%. Note that in Example 2 the Xylene loss is high at the high temperature employed despite the fact the contact time is one-half that of Example 1. Examples 1 and 2 illustrate that in order to eiect a large conversion of orthoxylene to meta-xylene and para-xylene, high temperatures are required, but that'large xylene losses are sustained when the requisite high temperatures and/or low space velocities are employed. 'Y

A comparison of Examples 3 and 4 shows that when the same feed (95% meta-xylene) is isomerized at 800 F. and at 1000 P. with all other conditions being identical, that at thev higher temperature the xylene losses to disproportionation increase from less than 1% at 800 F. to 11.5% at 1000 F. When isomerizing predomiproach to para-xylene equilibrium with meta-xylene rich feeds than with ortho-xylene rich feeds and that xylene losses are relatively low at lower isomerization temperatures. Y Pursuant to the invention, the desired high percentage of para-Xylenc approach to equilibrium with a minimum of xylene losses is attained by isomerizing the metaand para-xylene overhead fraction by contacting it with a silica-alumina catalyst at temperature in the range 700- 1000" F. and a space velocity in the range of 0.5-5.0,

` and separately isomerizing the ortho-xylene rich bottoms fraction by contacting it with the same catalyst at a temperature in the range about 9001050 F. and at a space velocity in the range 0.5-5 .0. The space velocity is increased Within the specified range as temperatures are increased within the temperature ranges set forth for use in isomerizing the two xylene fractions. Thus, in isomerizing the meta-xylene rich overhead fraction a space velocity of 0.5 to 1.5 is employed when the temperature is 700 F. and a space velocity of 3 to 5 is employed when the temperature used is toward 1000o F. Similarly, a space velocity of 0.5 to 1.5 is emloyed in isomerizing the ortho-xylene rich fraction at 900 F., while a spaceyelocity of 3 to 5 is employed when the temperature of isomerization is toward the upper limit of the described range. Y

From the two isomerization zones, the two isomerized mixtures are further processed by any one or combination ofthe following alternative methods:

(l) All, or a substantial part of the isomerized product I l from both fractions, is returned to the crystallization zone where portions of the para-xylene produced in the two isomerization zones is crystallized out of the mixture and thenvseparated by filtration.

(2) The .product from the isomerization of the overhead is returned to the crystallization zone and the product from the isomerization of the bottoms is fractionally distilled to separate three fractions; an overhead fraction comprising predominantly of ethylbenzene and lighter materials, a side cut fraction comprising meta-xylene and para-xylene which is returned to the crystallization zone,

and a bottoms fraction of predominantly ortho-xylene.

(3) The total product from both isomerization zones is passedinto a distillation zone where a separation of ethylbenzene and lighter materials as an overhead, metaxylene and para-xylene together as a side cut, and orthoxylene as a bottoms fraction is attained by fractional distllation. The meta, para-xylene side cut is returned to the crystallization zone to remove portions of the paraxylene, and the ortho-xylene bottoms fraction is returned to the bottoms isomerization zone.

In amodication of the present invention the original Xylene feed is passed into a distillation zone, the over;y

head fraction, comprised substantially of meta-xylene and para-xylene, is passed into the crystallization zone, the ortho-xylene rich bottoms fraction is passed into an isomerization zone, and the isomerized product is returned to the distillation zone with the original feed.

The following examples of feeds to the noted various zones are merely illustrative and the process of the present invention is not confined to the compositions shown.

(1) Illustrative feed to the crystallization zonewith no (2) Representative composition of a meta-para-xylene rich fraction separately subjected to isomerization at a temperature in the range 700 F.-l000 F.

Weight Compound Weight Percent Percent (Xylene basis) Ethylbenzene 8. 4 Para-xylene 9. 5 11. 9 Meta-xyleue 56. 6 70. 8 Ortho-xylene 13. 8 17. 3 C-f-Cg and Parans 11. 7

(3) Representative composition of an ortho-xylene rich fraction separately subjected to isomerization at a temperature in the range 900 F.-1100 F.

Weight Compound Weight Percent Percent (Xylene basis) Ethylhonve'ne 0 4 Para-xylene 5. 2 5. 5 Meta-xylene 16. 1 17. 1 Ortho-xyleue 72. 8 77. 4 Cr-C and Parans 5. 5

The appendedV drawing is a diagrammatic illustration of apparatus and process ow suitable for the practice of the invention.

A xylene-containing feed as described above is passed through line 1 into crystallization zone 2, where a portion of the para-Xylene present in-the feed is crystallized at a` temperature in the range 75 F. to -l20 F. The mixture containing para-Xylene crystals is removed from crystallization zone 2 by line 3 and is passedinto centrif ugal filter 4 'where the cooled mixture is centrifugally filtered to separate the crystalline phase and a mother liquor. The crystal phase, containing para-xylene at a substantially greater concentration than that of the mother liquor, is removed from centrifuge 4 by line 5 and the mother liquor, decient in vpara-xylene, is removed from centrifuge 4 by line 6. The crystallization is desirably Y..6 ethylbenzene and a bottom's fraction substantially cornposed of ortho-xylene. The overhead product is removed from distillation zone 7 by line 8, reilux condenser 9,

line 10, reux line 11, and line 12. Reflux ratios on distillates of from about 2:1 to about 12:1 have been found satisfactory. The ortho-xylene rich bottoms frac tion is removed from distillation zone 7. by line 13.

The overhead fraction is introduced into isomerization zone 14 by line 12 where it is isomerized by contacting it with a silica-alumina catalyst in the range about 700 F.-V 1000 F. as heretofore described. vThe predominantly ortho-xylene bottoms fraction from distillation zone 7 is passed by line 13 into isomerization zone 15 where it is isomerized with a' silica-alumina catalyst at a temperature in the range about 900 1050 F.

The isomerized overhead fraction is removed from isomerization zone 14 by line 16 and the isomerized bottoms fraction is removed from isomerization zone 15 by line 18 and are introduced into distillation zone 19. In

distillation zone 19, the following separations are made by eicient fractionation:

Ethylbenzene and lighter components are passed through line 20 into reflux condenser 21, and portions of the ethylbenzene and lighter components are returned to distillation zone 19 as reflux by lines 22 and 23, and the remaining portion is passed to storage facilities by line 24. A fraction of predominantly metaand paraxylene is' removed from line 25. A fraction predominantly ortho-xylene is removed from distillation zone 19 by line 26.v A heavy bottoms product is removed by line 27.

The metaand para-xylene stream is passed from distillation zone 19 by lines 25 and 1 intothe crystallization zone 2 and the orthoxylene rich fraction from distillation zone 19 is passed through lines 26 and 13 into isomerization zone 15. Y.

y Asa-noted hereinbefo're, the above-described process can be modified by passing all, or a substantial part, of the isomerized products from isomerizationzones 14 and 15 to crystallization zone 2 by lines 16, 17, 18, 28, 25 and 1. By operating in this manner, portions of the para-xylene produced in the two isomerization zones can be crystallized out of the isomerized products.

`It is readily'apparent from the foregoing description that various modifications of the process can be made within the spirit of the present invention and the scope of the appended claims. For the sake of simplicity and clarity, apparatus has not been shown in detail in the drawings but is illustrated only as to major unit operation in the process. Many detailed pumps, valves, condensers, heat exchangers, temperature controls, and the like have been omitted,since any suitable form of apparatus incorporating these features can be supplied in obvious manner by those skilled in the art.

We claim: ,l

1. A process for producing para-xylene which comprises cooling a xylene feed to a temperature in the range conducted in the manner described in'U. S. Patent No.

- F. to 120 F. to cause crystallization of a portion of the para-xylene, separating the para-Xylene crystals and a mother liquor, fractionally distilling the mother liquor to separate an overhead fraction comprising metaxylene and para-Xylene and having an ortho-xylene content substantially less than that of the mother liquor and a bottoms fraction having an ortho-xylene content subs tantiall'y higher than that of the mother liquor, isomerizing the overhead fraction by contacting it with a silicaalumina catalyst at 700 F. to l000 F. and at a space velocity in the range from 0.5 to 5.0 v./v./hr., separately isomerizing the bottoms fraction by contacting it with a silica-alumina catalyst at conditions of temperature and space velocity substantially more severe than those employed in isomerizing thevoverhead fractions, the temperature being in the range from 900 F. to l050 F. and the space velocity being in the range from 0.5 to 5.0 v./v./hr., passing the isomerizing product mixture into a fractional distillation zone to separate an overhead fraction comprising meta-xylene and para-xylene and having an ortho-xylene content substantially less than that of joined isomerizing product mixtures and a bottom fraction havingan ortho-xylene content substantially higher than that of the joined isomerizing product mixtures, and fractionally crystallizing the overhead mixture to recover para-xylene.

2. A process for producing para-xylene which comprises cooling a hydrocarbon fraction consisting preg dominantly Tof xylenes including ortho-xylene, meta- Xylene, and para-xylene in a crystallization zone to a temperature in the range from -75 F. to` 120 F. to produce a slurry containing para-xylene crystals, ltering the slurry to separate solidpara-xylene crystals and a liquid filtrate, fractionally distilling the ltrate to separate an overhead fraction rich in meta-xylene and having an ortho-xylene content substantially lower than that of the filtrate and a bottoms fraction rich in ortho-xylene and having a substantially higher content of ortho-xylene than the filtrate, contacting the overhead fraction with a silica-alumina catalyst at a temperature in the range from 700 F. to 1000 F. andata space velocity in the range from 0.5 to 5,0 v./v./hr., separately contacting the bottoms fraction with a silica-alumina catalyst at a temperature in the range from about 850 F. to 1050 F. and at a space velocity in the range from 0.5 to 5.0 v./v./hr., said bottoms fraction being contacted with said catalyst at a higher temperature than the catalyst'contacting step of said overhead fraction, the space' velocities withinV the dened ranges being coordinated with the temperatures Within the defined ranges by employing lower space velocities within the space velocity ranges in conjunction with lower temperatures within the defined temperature ranges, and returning a substantial proportion of the isomerization products to the crystallization zone.

3.` A process for isomerizing a xylene feed consisting predominantly of ortho-xylene and meta-xylene and containing para-xylene in less than equilibrium amount which comprises fractionally distilling the xylene feed to separate an overhead fraction comprising meta-xylene and para-xylene and having an ortho-xylene content substantially less than that of the feed and a bottoms fraction having an ortho-xylene content substantially higher than that of the feed, contacting the overhead fraction with a silica-alumina catalyst at a temperature in the range from 700 F. to 1000 F. and at a space velocity in the range from 0.5 to 5.0 v./v./hr., separately con tacting the bottoms fraction with a silica-alumina catalyst at Ia temperature in the range from about 850 F. to 1050 F. and at a space velocity in the range from 0.5 to 5.0 v./v./hr., said bottoms fraction being contacted with said catalyst at a higher temperature than the catalyst contacting step of said overhead fraction, the space velocities within the defined ranges being coordinated with the temperatures within the defined ranges by employing lower space velocities within the space velocity ranges in conjunction with lower temperatures Within the defined temperature ranges.

4. A process for isomerizing a xylene feed consisting predominantly of ortho-xylene and meta-xylene and containing para-xylene in less than equilibrium amount which comprises fractionally distilling the xylene feed to separate an overhead fraction comprising meta-xylene and para-xylene and having an ortho-xylene content substantially less than that of tion having, an ortho-xylene content substantially higher than that of the feed, contacting the overhead fraction with a silica-'alumina catalyst at a Ytemperature in the range from 700 F. to-1000 F. and at a space velocity in the range from 0.5 to 5.0 v./v./hr., separately contacting ,the bottoms fraction with a silica-alumina Vcatalyst at a temperature in the range from about 850 F. to l050 F. and at a space velocity in the range from 0.5 to 5.0 v./v./hr., said bottoms fraction being contacted with said catalyst at a higher temperature than the( catalyst contacting step of vsaid overhead fraction, the space 4velocities within the delined ranges being coordinated with the temperatures within the defined ranges by employing lower space velocities within the space velocity ranges in conjunction with lower temperatures within the defined temperature ranges, passing the two isomerized product mixtures into a distillation zone, separating a meta-xylene and para-xylene rich overhead fraction from an ortho-xylene rich bottoms fraction, passing the overhead fraction into a fractional crystallization zone and recovering para-xylene crystals from said crystallization zone.

5. A process for isomerizing a xylene feed consisting predominantly of ortho-xylene and meta-xylene and containing para-xylene in less than equilibrium amount which comprises fractionally distilling the xylene feed to separate Aan overhead fraction comprising meta-xylene and para-xylene and having an orthoxylene content substantially less than that of the feed and a bottoms fraction having an ortho-xylene content substantially higher than that of the feed, contacting the overhead lfraction with asilica-alumina catalyst at a temperature in the range from 700 F. to 1000 F. vand at a space velocity in the range from 0.5 to 5.0 v./ v./ hr., separately contacting the bottoms fraction with a silica-alumina catalyst at a temperature in the range from about 850 F. to l050 F. and at a space velocity in the range from 0.5 to 5.0 v./v./hr., said bottoms fraction being contacted with said catalyst at `a higher temperature than the catalyst contacting step of said overhead fraction, the space velocities within the defined ranges being coordinated with the temperatures within the dened ranges by ern-l ploying lower space velocities within the space velocity ranges in conjunction with lower temperatures within the defined temperature ranges, passing the two isomerized product mixtures into a distillation zone, separating a meta-xylene and para-xylene rich overhead fraction from an ortho-xylene rich bottoms fraction, passing the overhead fraction into a fractional crystallization zone and recovering para-xylene crystals from 4said crystallization zone, and returning the ortho-xylene rich bottoms fraction to an isomerization zone and contacting the bottoms fraction with a silica-alumina catalyst at a temperature in the range from about 850 F. to 1050 F. and at a space velocity in the range from 0.5 to 5.0 v./v./hr.

References Cited in the file of this patent UNTED STATES PATENTS 2,403,757 Reeves July 9, 1946 2,532,276 Birch et al Dec. 5, 1950 2,564,388 Bennett let al Aug. 14, 1951 2,632,779 Pfennig Mar. 24, 1953 2,653,175 Davis Sept. 2 2, 1953 the feed and a 'bottoms frac- 

1. A PROCESS FOR PRODUCTING PARA-XYLENE WHICH COMPRISES COOLING A XYLENE FEED TO A TEMPERATURE IN THE RANGE -75*F. TO -120*F. TO CAUSE CRYSTALLIZATION OF A PORTION OF THE PARA-XYLENE, SEPARATING THE PARA-XYLENE CRYSTALS AND A MOTHER LIQUOR, FRACTIONALLY DISTILLING THE MOTHER LIQUOR TO SEPARATE AN OVERHEAD FRACTION COMPRISING METAXYLENE AND PARA-XYLENE AND HAVING AN ORTHO-XYLENE CONTENT SUBSTANTIALLY LES THAN THAT OF THE MOTHER LIQUOR AND A BOTTOMS FRACTION HAVING AN ORTHO-XYLENE CONTENT SUBSTANTIALLY HIGHER THAN THAT OF THE MOTHER LIQUOR, ISOMERIZING THE OVERHEAD FRACTION BY CONTACTING IT WITH A SILICAALUMINA CATALYST AT 700*F. TO 1000*F. AND AT A SPACE VELOCITY IN THE RANGE FROM 0.5 TO 5.0 V./V./HR., SEPARATELY ISOMERIZING THE BOTTOMS FRACTION BY CONTACTING IT WITH A SILICA-ALUMINA CATALYST AT CONDITIONS OF TEMPERATURE AND SPACE VELOCITY SUBSTANTIALLY MORE SEVERE THAN THOSE EMPLOYED IN ISOMERIZING THE OVERHEAD FRACTIONS, THE TEMPERATURE BEING IN THE RANGE FROM 900*F. TO 1050*F. 